Today's semiconductor technology is rapidly forcing device sizes below the 0.5 micron level, even to the 0.25 micron size. With device sizes on this order, even higher precision is being demanded of the processes which form and shape the devices and the dielectric layers separating the active devices. In the fabrication of semiconductor components, the various devices are formed in layers upon an underlying substrate typically composed of silicon, germanium, or gallium arsenide. The various discrete devices are interconnected by metal conductor lines to form the desired integrated circuits. The metal conductor lines are further insulated from the next interconnection level by thin films of insulating material deposited by, for example, CVD (Chemical Vapor Deposition) of oxide or application of SOG (Spin On Glass) layers followed by fellow processes. Holes, or vias, formed through the insulating layers provide electrical connectivity between successive conductive interconnection layers. In such microcircuit wiring processes, it is highly desirable that the insulating layers have a smooth surface topography, since it is difficult to lithographically image and pattern layers applied to rough surfaces.
One semiconductor manufacturing process, chemical/mechanical polishing (CMP), is used to provide the necessary smooth semiconductor topographies. CMP can be used for planarizing: (a) insulator surfaces, such as silicon oxide or silicon nitride, deposited by chemical vapor deposition; (b) insulating layers, such as glasses deposited by spin-on and reflow deposition means, over semiconductor devices; or (c) metallic conductor interconnection wiring layers. Semiconductor wafers may also be planarized to: control layer thickness, sharpen the edge of via "plugs," remove a hardmask, remove other material layers, etc. Significantly, a given semiconductor wafer may be planarized several times, such as upon completion of each metal layer. For example, following via formation in a dielectric material layer, a metallization layer is blanket deposited and then CMP is used to produce planar metal studs.
Briefly, the CMP process involves holding and rotating a thin, reasonably flat, semiconductor wafer against a rotating polishing surface. The polishing surface is wetted by a chemical slurry, under controlled chemical, pressure, and temperature conditions. The chemical slurry contains a polishing agent, such as alumina or silica, which is used as the abrasive material. Additionally, the slurry contains selected chemicals which etch or oxidize selected surfaces of the wafer to prepare them for removal by the abrasive. The combination of both a chemical reaction and mechanical removal of the material during polishing, results in superior planarization of the polished surface. In this process it is important to remove a sufficient amount of material to provide a smooth surface, without removing an excessive amount of underlying materials. Accurate material removal is particularly important in today's submicron technologies where the layers between device and metal levels are constantly getting thinner.
One problem area associated with chemical/mechanical polishing is in the area of slurry consistency. The polishing slurry is a suspension of a mechanical abrasive in a liquid chemical agent. The mechanical abrasive, typically alumina or amorphous silica, is chosen having a design particle size specifically to abrade the intended material. The desired particle size is chosen in much the same way that a sandpaper grade is chosen to give a particular smoothness of finish on wood, metal, or paint. If the particle size is too small, the polishing process will proceed too slowly or not at all. However, if the particle size is too large, desirable semiconductor features may be significantly damaged. Unfortunately, because the slurry is a suspension, the abrasive particles in the slurry have a tendency to agglomerate, forming relatively large clumps when compared to semiconductor device sizes. While these clumps of abrasive can grow to significant size, e.g., 0.1 .mu.m to 30 .mu.m, depending in part upon their initial abrasive particle size, they retain their ability to abrade the semiconductor wafer surface. The agglomeration problem is most apparent when the slurry is allowed to stand. If the slurry is allowed to stand in the supply line for any appreciable time, the agglomeration begins and sometimes clogs the supply line. This results in the need to stop the processing and flush the supply line. Of course, once the supply line is flushed, the stabilized slurry must be reflowed through the line, forcing any residual water from the line. This entire process is time consuming and ultimately very expensive when the high cost of the wasted slurry and the lost processing time is considered. Agglomeration is especially a problem in metal planarization slurries.
To help alleviate this agglomeration problem, the conventional approach has been to keep the slurry flowing in a loop and to perform a coarse filter of the slurry while it is in the loop. To supply the slurry to the polishing platen, the loop is tapped, and the slurry is subjected to a point-of-use, final filter just before it is applied to the polishing platen. However, as the final filter strains out the larger particles, the filter becomes clogged, raising the flow pressure required and necessitating a filter change or cleaning operation. The increased pressure may deprive the polishing platen of slurry and endanger the planarization process. Cleaning or changing the filter clearly interrupts the CMP processing. Naturally, cleaning or replacing the filter is both time consuming and costly. Further, as the filters are extremely fine (capable of passing particles less than about 10 .mu.m to 14 .mu.m in size), the filters themselves represent a significant cost. Additionally, when the processing is stopped to clean/replace the filter, the slurry supply line must be flushed with water to prevent even more agglomerate from forming. This flushing water initially dilutes the slurry when processing resumes, further delaying the CMP process. Unfortunately, even when the filters are flushed regularly, the filters may only last for a period of a few days or even hours, depending upon the daily processing schedule. Furthermore, these filters still allow particles that have particle sizes larger than the intended design particle size to reach the polishing surface.
Accordingly, what is needed in the art are a slurry delivery system and method of use thereof which efficiently breaks up the CMP slurry agglomerate, and returns the slurry particulate matter substantially to the design particle size.